In the rotary drilling of wells, such as hydrocarbon (oil and gas) wells, a mud is continuously circulated from the surface down to the bottom of the hole being drilled and back to the surface again. The mud--usually a fluid mixture of a clay such as bentonite suspended in a continuous phase such as water--has several functions. One of these is to transport the cuttings drilled by the drill bit up to the surface where they are separated from the mud. For this purpose the mud must be viscous enough to entrain the cuttings yet fluid enough to pump. Another function is to impose an hydrostatic pressure on the walls of the borehole so as to avoid a collapse of the borehole and an influx of gas or liquid from the formations being drilled. For this function the mud must be dense enough to resist formation pressure, yet not so dense that its pressure forces it deep into the formations, possibly fracturing them. It is therefore important to monitor the characteristics of the mud, and to keep them within certain limits. Weighting materials, barite for example, are added to the mud to make it exert as much pressure as needed to contain the formation pressures. Clay is added to the mud so as to keep the drilled cuttings in suspension as they move up the hole. The clay also sheathes the wall of the hole (this thin layer of clay, called mud cake, forms a permeability barrier, and prevents or reduces fluid loss). Numerous chemicals are available to give the mud the exact properties it needs to make it as easy as possible to drill the hole, and the importance of the mud, and the difficulties of controlling its composition, can be further appreciated from the following additional comments.
Maintaining the stability of the borehole is one of the major problems encountered in drilling oil and gas wells. It has been observed in the field that holes in shale sections frequently go out of gauge, due to loss of material from the borehole wall. This material can become detached from the wall in the form of large fragments (cavings), which are normally carried to the surface by the circulating mud, just as the drilled cuttings are. However, if the hole-cleaning capacity of the mud is insufficient, cavings collect on ledges, and may cause the drill pipe to stick on pulling out of the hole. The necessity to re-drill through fill accumulated on the bottom of the hole during trips is another result of the process.
Moreover, regardless of the efficiency of the cavings removal, in all cases there is inevitably a gradual build-up of dispersed particles in the mud, which particles are too fine to be removed by the solids control equipment. This may give rise to a host of secondary problems. For instance, the increased solids content slows down the drilling rate, and, as drilled solids form a poor filter cake, problems in controlling the fluid loss may cause differential sticking on permeable sands. In addition, there may be difficulty controlling the mud weight, leading to lost circulation, and an unstable rheology.
In some circumstances, shales swell in contact with the mud in such a way that the well bore diameter decreases. In such cases, identified in the field by a need for frequent reaming, the well bore closes down on to the drill string, and there is once again an increased risk of pipe sticking.
The various forms of hole instability resulting from the interaction between the drilling fluid and the subterranean formations penetrated by the borehole are related to the hydration and dispersion of the clay sediments.
It is known that during the drilling process the ionic composition of the drilling mud changes from its original formulation. These changes in composition are in part a measure of downhole processes which may be termed mud-rock interactions. An important example of mud-rock interactions is ion exchange between cations in the mud and in shale formations. Until recently drilling practice has not required the ionic composition of the mud to be monitored, so that the extent of these interactions has not been determined, and the composition of the drilling mud has not been accurately maintained. However, in the Specification of our co-pending Application for European Patent No: 88/301,856.6, we have described how important such a monitoring process is, and how useful it can be. In general, in that Specification we described a method for controlling the drilling of boreholes by determining the ionic compositions of the drilling muds and/or drilled cuttings in order to monitor various chemical processes which occur in the well bores, e.g. salt water influxes, changes in the solubility of salts with changes in pH, and cation exchange processes involving the cations added to the water-base mud (e.g. potassium, calcium) to stabilise shale sections.
The various general and preferred methods of the earlier invention have proven useful in the control of drilling mud composition, and yet our further research has indicated that there may nevertheless be a number of problems. For example, the separation of the liquid part of the mud by filtration has not always been easy, for, dependant on the mud's type, its clay components often fulfil only too well their intended purpose (to sheathe the well bore wall with the thin, impermeable "mud cake" layer), and in just the same way form an almost impenetrable layer in the filter apparatus, so reducing the flow of liquid therethrough to almost nothing, even when filtering under several atmospheres pressure. Again, the filter techniques may not always satisfactorily remove the fines (the very small particles generated, for instance, by the drilling procedure), and if the fines in a mud sample dissolve on dilution of the sample, as will carbonate fines, then this can seriously affect the ionic concentrations, and so give rise to significantly misleading analytical results.